FIG. 6 is a cross-sectional view illustrating a prior art forward-mesa ridge type 0.78 .mu.m wavelength semiconductor laser having a real refractive index guided structure (hereinafter referred to as forward-mesa ridge type semiconductor laser). In the figure, reference numeral 1 designates an n type GaAs substrate. An n type Al.sub.x Ga.sub.1-x As (x=0.48) lower cladding layer 2 having a thickness of about 1.5 .mu.m is disposed on the n type GaAs substrate 1. An AlGaAs quantum-well active layer 3 is disposed on the lower cladding layer 2. A p type Al.sub.x Ga.sub.1-x As (x=0.48) upper cladding layer 4 having a thickness of about 1.5 .mu.m is disposed on the quantum-well active layer 3, and the upper cladding layer 4 includes a mesa ridge stripe 4a. An n type Al.sub.x Ga.sub.1-x As (x=0.70) current blocking layer 6 having a thickness of about 1.5 .mu.m is disposed adjacent both sides of the mesa ridge stripe 4a. A p type GaAs contact layer 5 having a thickness of about 3 .mu.m is disposed on the upper cladding layer 4 and the current blocking layer 6. A stripe-shaped p side electrode 11 is disposed on the contact layer 5 over the mesa ridge stripe 4a, and an n side electrode 12 is disposed on a rear surface of the substrate 1.
FIGS. 7(a)-7(d) are cross-sectional views illustrating process steps in a method of fabricating the forward-mesa ridge type semiconductor laser. In these figures, the same reference numerals as shown in FIG. 6 designate the same or corresponding parts. Reference numeral 8 designates a selective growth mask. The fabricating method will be described below.
As shown in FIG. 7(a), the lower cladding layer 2, the active layer 3 and the upper cladding layer 4 are successively grown on the substrate 1 by MOCVD (metal organic chemical vapor deposition) to form a double heterojunction structure.
After an SiN layer is formed on the upper cladding layer 4, patterning is performed by conventional lithography to form the SiN mask 8. Then, using the SiN mask 8 as a mask, portions of the upper cladding layer 4, except for an active region 7, are etched to have a thickness of 0.1 to 0.4 .mu.m, thereby forming the mesa ridge stripe 4a (FIG. 7(b)).
As shown in FIG. 7(c), the current blocking layer 6 is grown by MOCVD on the upper cladding layer 4 to cover both sides of the mesa ridge stripe 4a.
As shown in FIG. 7(d), after removing the SiN mask 8, the contact layer 5 is grown on the mesa ridge stripe 4a and the current blocking layer 6. Finally, the p side electrode 11 is formed on the contact layer 5 and the n side electrode 12 is formed on the rear surface of the substrate 1, completing the semiconductor laser.
As described above, in the prior art forward-mesa ridge type semiconductor laser, the n type Al.sub.x Ga.sub.1-x As (x=0.70) current blocking layer 6 having a refractive index smaller than that of the upper cladding layer 4 is disposed adjacent both sides of the mesa ridge stripe 4a of the upper cladding layer 4. Therefore, a current is efficiently concentrated in the active region 7. Further, since an equivalent refractive index of the active region 7 is higher than that of other portions of the semiconductor laser, light emitted from the active region 7 is efficiently confined in the active region 7. Consequently, laser oscillation is produced at a low current density.
FIG. 8 is a cross-sectional view illustrating a prior art reverse-mesa ridge type semiconductor laser. The reverse-mesa ridge type semiconductor laser is fundamentally identical to the forward-mesa ridge type semiconductor laser, except that the mesa ridge stripe 4a has a reverse mesa shape. The forward-mesa ridge type semiconductor laser has advantages over the reverse-mesa ridge type semiconductor laser in that the side etching is less and the ridge width is more easily controlled when the mesa ridge stripe is formed, and that the current blocking layer 6 is more easily buried.
In the prior art forward-mesa and reverse-mesa ridge type semiconductor lasers, however, because the current blocking layer 6 having a lattice constant larger than those of the substrate 1 and the upper cladding layer 4 is formed in the vicinity of the active region 7, stress is applied to the active region 7, so that crystalline defects due to the stress tend to be produced. Especially in the forward-mesa ridge type semiconductor laser, since the current blocking layer 6 is close to the active region 7, as compared with the reverse-mesa ridge type semiconductor laser, it is impossible to ensure the reliability of the semiconductor laser. Consequently, in order to prevent deterioration of reliability due to the stress applied to the active region 7 while maintaining the advantages of facilitation of the ridge width control and the burying growth, it is necessary to adopt the forward-mesa ridge stripe structure for a semiconductor laser and not to form a current blocking layer having a large Al composition in the vicinity of the active region 7.
In addition, in the prior art forward-mesa ridge type semiconductor laser, the current blocking layer 6 comprises n type AlGaAs semiconductor having a large Al composition. In the n type AlGaAs semiconductor having a large Al composition, it is difficult to dope the AlGaAs semiconductor with an n type dopant impurity, and it is impossible to make its n type carrier concentration high. Therefore, in order to satisfactorily exhibit the current blocking effect, the thickness of the current blocking layer 6 must be not less than 1 .mu.m. For this reason, its thickness is 1.5 .mu.m. Therefore, as the current blocking layer 6 becomes thicker, the stress applied to the active region 7 is increased, whereby the reliability of the semiconductor laser is deteriorated. Consequently, in the prior art forward-mesa ridge type semiconductor laser, high output power operation and high reliability of the laser are incompatible with each other.